A Long-Term Evolution (LTE) system offers high peak data rates, low latency, improved system capacity, and low operating cost resulting from simple network architecture. An LTE system also provides seamless integration to older wireless network, such as GSM, CDMA and Universal Mobile Telecommunication System (UMTS). Enhancements to LTE systems are considered so that they can meet or exceed International Mobile Telecommunications Advanced (IMT-Advanced) fourth generation (4G) standard. One of the key enhancements is to support bandwidth up to 100 MHz and be backwards compatible with the existing wireless network system. Carrier aggregation (CA) is introduced to improve system throughput. With carrier aggregation, the LTE-Advanced system can support peak target data rates in excess of 1 Gbps in the downlink (DL) and 500 Mbps in the uplink (UL). Such technology is attractive because it allows operators to aggregate several smaller contiguous or non-continuous component carriers (CC) to provide a larger system bandwidth, and provides backward compatibility by allowing legacy users to access the system by using one of the component carriers.
Orthogonal frequency division multiplexing (OFDM) radio technology has been incorporated into LTE/LTE-A because it enables high data bandwidth to be transmitted efficiently while still providing a high degree of resilience to reflections and interference. In OFDM communication systems, the transmit power of each mobile station (UE) needs to be maintained at a certain level and regulated by the network. The maximum transmit power of each UE, however, is different depending on UE capacity. Power headroom report (PHR) is a mechanism to configure the UE to provide its power capacity and usage to the network. A UE uses PHR mechanism to periodically provide its serving base station (eNB) with its power headroom (PH), which is defined as a power offset between a UE-configured maximum transmit power and a UE-calculated current UE transmit power. Based on the received PH information, the eNB can regulate the UE transmit power with proper resource allocation.
FIG. 1 (Prior Art) illustrates a power headroom (PH) and other related parameters of a UE in LTE Rel-8/9 systems without carrier aggregation. The PH value of the UE is defined in Eq. (1), while the UE-configured maximum output power PCMAX is defined in Eq. (2):PH=PCMAX−UE transmit Power  (1)PCMAX_L<=PCMAX<=PCMAX_H  (2)where                PCMAX_L=MIN {PEMAX−ΔTC, PPOWERCLASS-MPR-A-MPR-ΔTC}        PCMAC_H=MIN {PEMAX, PPOWERCLASS}        PEMAX is configured by higher layers        PPOWERCLASS is the maximum UE output power        Maximum Power Reduction (MPR): the maximum allowed reduction of maximum power of certain modulation order and the number of resource blocks        Additional Maximum Power Reduction (A-MPR): the maximum allowed reduction of maximum power for the number of resource blocks and the band        ΔTC=1.5 dB when the CC at the edge of a band; 0 dB otherwise        
FIG. 2 (Prior Art) illustrates multiple power headroom values and other related parameters of a UE in LTE Rel-10 systems with carrier aggregation. In LTE Rel-10 systems, more flexible resource assignments are required to support advanced features including carrier aggregation, simultaneous PUCCH (Physical Uplink Control Channel) and PUSCH (Physical Uplink Shared Channel) transmission, parallel transmission of PUSCHs on multiple CCs, multi-clustered PUSCH, and power scaling. In the example of FIG. 2, the UE is configured with two component carriers CC1 and CC2. The UE-configured maximum output powers for CC1 (e.g., PCMAX,C1) and for CC2 (e.g., PCMAX,C2) depend on upper layer configurations (e.g., PMAX_CC1 and PMAX_CC1) and other CC-related parameters such as MPR, A-MPR, and ΔTC. Furthermore, because CC1 and CC2 belong to the same UE, and are served by the same or different power amplifier(s), the total maximum output power of both CC1 and CC2 may be limited to additional constraints such as PMAX_UE or PMAX_PA. As a result, multiple PH values need to be reported to the eNB for UE transmit power control. Therefore, the existing PHR mechanism for Rel-8/9 systems without CA is no longer adequate to consider various transmit power limitations imposed on multiple configured CCs of a UE, on power amplifiers that serve the CCs, and on the UE.